The bonding process has the finest layer of adhesive between the structure and the strain gauge itself that makes the actual measurement. Alternatively, we can spot weld the gauges onto the structure, where permitted, but because we first fit the gauge to a steel plate, before welding onto the structure itself, this technique adds another layer between the item to be measured and the ‘sensor’. Here we make a comparison between glued gauges and spot welded gauges.
HBM’s FiberSensing subsidiary in Porto tested the effect on measurement accuracy, with the results in the graph below. Strain gauge type LS31 is a resistive gauge, welded onto side A of a sample structure, FS62 is an optical gauge, welded onto the same side. On the reverse of the same sample we have two bondable (glued) strain gauges, LY41 is resistive, and FBG is optical.
The fiber-optical gauges are configured in arrays and there are advantages and disadvantages to this. The array (or cable) runs in a loop from DAQ to measurement position, and back, with a number of measurement points along its length. This, in large structures such as offshore wind turbine foundations, means much less cable is required, bringing significant cost benefits. In the event of a single cable break in one array, it is sometimes possible to continue to measure, by sending the light from the other end of the array. However, resistive gauges, wired individually, offer less risk of losing an entire section of the measurement chain in the event of a single cable break.
From a calibration perspective, the HBM fiber-optical interrogator has an in-built calibration reference, self-checking before measurement to aid long-term accuracy.
|Resistive strain gauge, compared to optical||Optical strain gauge, compared to resistive||Note|
|DAQ hardware cost||-10%||+10%|
|Strain gauge cost||-75%||+75%||Optical strain gauges include cable as part of array. Resistive gauges require extra cable, hence the large cost difference.|
|DAQ Installation cost||Same||Same|
|Strain gauge installation cost by qualified Engineer||+15%||-15%|
|Cabling||3km||Included in strain gauge cost as part of array|
|Total difference in cost for an installed system||c.-10%|
The use of fiber-optical technology is not new. Despite this, there had been a relatively slow uptake in its use for the application of offshore wind turbine foundation monitoring. In an industry requiring a design life of perhaps 25 years, with strain gauges often installed in a position unreachable after installation, there can be no room for experimentation.
The technical benefits have been clear for many years, but to be commercially viable this technology must bring considerable cost benefits, too. The asset owner looking to extend operational life will appreciate the benefit of continued accurate measurement after many years, as will engineers looking for reliable data for design validation. But the benefit of a lower installed cost, at time of construction, now makes fiberoptical technology, when installed correctly, a cost effective solution for accurate foundation monitoring in offshore wind turbines.
Please contact HBM for the contents of the tests mentioned here, technical datasheets on the products mentioned, or advice on how to ensure longevity in subsea measurement points.
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